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What Is the Ferranti Effect?

By Jo Dunaway
Updated May 23, 2024
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The Ferranti effect is a term that describes an electrical phenomenon that causes voltage to rise at the end of an alternating current (AC) transmission line. This is likely to occur when a transmission line is either very lightly loaded or it is interrupted due to a power outage. The Ferranti effect is named for the electrical engineer Sabastian Zianni de Ferranti, who designed the power station and distribution network for an AC facility in England in 1887, shortly after the War of the Currents. The War of the Currents was the debate between Nikola Tesla, who advocated the use of AC power distribution, and Thomas Edison, who advocated the use of direct current (DC) power distribution. The new construction of the Deptford, England power station first exhibited the Ferranti effect in its AC distribution lines when opened in 1891.

Observances of the Ferranti effect happen more frequently along buried transmission lines in contact with earth, as at the Deptford power station, rather than along transmission lines strung overhead. Burying transmission lines contributes to heightened load as it nears the end of its line; and for this reason shunt reactors must be applied to underground lines to stabilize the power flow. As the desired distribution is for stepped-down power voltage at the receiving end of transmission lines versus the raw power voltages at the sending source, the Ferranti effect must be compensated for along the line in order not to avoid delivering excessively high voltage to household or industrial appliances.

The length of the transmission line is also a contributor to the Ferranti effect. Transmission lines produce inductive charges when their load is light, and as these charges build, capacitors must be attached along the length of the lines at intervals to step the power back down. Using shunt reactors and capacitors in conjunction, and switching on more capacitors along the line as the need arises, keeps voltages more in phase or synchronization between source and line ends.

In AC power distribution networks, power factor is a term standing for real power versus apparent power. Real power is the power in the transmission line that produces the working power to appliances on the receiving end. When compensating for Ferranti effect, the real power needs to be differentiated from apparent power, and the requisite amount of step-down of power should be applied through shunt reactors to provide the counter effect to build-up of voltage. Additionally, line engineers can use the electronic switching on and off of capacitors to make adjustments as needed when monitoring informs of Ferranti effect overloads.

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